An IC socket has been conventionally used for electrically connecting a lead terminal protruded from an IC to a circuit board. With the IC socket, the electrical conduction between an IC and a circuit board was conventionally established by soldering a contact pin protruded from an IC socket into the circuit board through a hole formed on the circuit board, or by directly soldering the contact pin to electric wire of the circuit board.
Here, the IC socket is an expendable item, and therefore may be needed to be replaced. In the replacement, the IC socket thus soldered into the circuit board needs to be melted again to be removed from the circuit board, which complicates the replacement process.
In the field of IC socket, there is a demand for an IC socket capable of electrical conduction between the IC and circuit board under a high temperature, for example, under 400° C. Such an IC socket is however not suitably connected to a general circuit board which is not heat resistant. A circuit board of ceramic may be suitable, but a ceramic circuit board is expensive and limited in size, approximately 200 mm square at the maximum. Further, even if a ceramic circuit board is used, it cannot be soldered under 400° C. Then, the circuit board needs to be welded by silver or electrically, but this is expensive because of the large number of connection portions in a narrow pitch. Moreover, an IC socket welded to a contact pin by silver or by an electrical manner will not be easily replaced.
To make the replacement easier, a particular IC socket (IC socket 100), which is shown in FIG. 15, is available in the market, which allows a lead terminal 112 protruded from an IC 111 to be electrically conducted to a pad 121 of a circuit board 120. The circuit board connected to this IC socket 100 is a metal board coated with ceramic or the like, and a contact pin is provided in contact with a pad printed on the ceramic surface.
More specifically, as shown in FIG. 15, a contact pin 102 made of a conductive material is buried in a through hole 101 which penetrates through the IC socket 100. The upper half of the contact pin 102 is a holding section 102a for holding the lead terminal 112 protruded from the IC 111. The lower half of the contact pin 102 is a projection section 102b which is protruded from the IC socket 100. The front edge of each of the projection sections 102b of plural contact pins 102 is brought into contact with a pad 121 of the circuit board 120 by connecting a bis 103 into the circuit board 120 through the IC socket 100. With this structure, the welding by silver or in an electrical manner is not necessary, and the IC socket 100 can be easily replaced.
However, in this arrangement, it is difficult to ensure the even contacts of the entire projection sections 102b into the pad 121 of the circuit board 120, and therefore may cause contact failure.
As one example of technique made in view of this problem, Japanese Laid-Open Patent Application Tokukai 2001-267029 (published on Sep. 28, 2001) discloses a structure in which the contact pin is given some elastic force by a spring so as to ensure its contact with the circuit board.
To more specifically explain the IC socket of Japanese Laid-Open Patent Application Tokukai 2001-267029 (published on Sep. 28, 2001), as shown in FIG. 16, the IC socket 200 includes a contact pin 230 in a socket body 201, and this contact pin 230 connects a solder ball 211 of an IC package 210 and a pad 221 of a circuit board 220.
The contact pin 230 includes a sleeve 231, a plunger 232, and a coil spring 233. The sleeve 231 has a large outer diameter section 231a which is enlarged in outer diameter. The plunger 232 is inserted into the sleeve 231 from the bottom of the sleeve 231, and vertically moves inside the sleeve 231. The coil spring 233 gives an upward elastic force to the sleeve 231 while giving a downward elastic force to the plunger 232. The sleeve 231 is vertically movable with respect to the socket body 201, and a ball contact section 234 on the upper section of the sleeve 231 is brought into contact with the solder ball 211 to establish electrical conduction therebetween. The lower end of the plunger 232 is brought into contact with the pad 221 of the circuit board 220 to establish electrical conduction therebetween.
With this arrangement, the IC socket 200 avoids the defect that some of the plural contact pins 230 fail to be brought into contact with the pad 221 of the circuit board 220, which would result in contact failure.
However, in the IC socket disclosed in Japanese Laid-Open Patent Application Tokukai 2001-267029 (published on Sep. 28, 2001), only a small portion (point) of the lower edge of the plunger 232 is in contact with the pad 221 of the circuit board 220; therefore, there still is a possibility of contact failure particularly in the case where the plunger 232 is deformed by heat, or in the case where an oxide film is generated between the pad 221 and the plunger 232.